My mom doesn't believe in Mother's Day. It's true.
This is the usual dialogue:
LINDSAY: Happy Mother's Day, Mom! What are you going to do today?
MOM: I don't believe in Mother's Day.
LINDSAY: Well...oh. Yeah. Well...
And I usually send her a card despite her non-belief because I, for one, feel like any chance to tell my mom how amazing she is (be it Sunday, May 9th or any other time in the year) is an opportunity not to be missed.